Method of removing sulfur dioxide from a flue gas stream

ABSTRACT

A method of removing SO 2  from a flue gas stream including SO 2  includes providing a source of trona and injecting the trona into the flue gas stream. The temperature of the flue gas is between about 600° F. and about 900° F. The trona is maintained in contact with the flue gas for a time sufficient to react a portion of the trona with a portion of the SO 2  to reduce the concentration of the SO 2  in the flue gas stream.

BACKGROUND

The present invention relates to the purification of gases, and moreparticularly to a method of purifying flue gases which contain noxiousgases such as SO₂.

Dry sorbent injection (DSI) has been used with a variety of sorbents toremove SO_(x) and other gases from flue gas. However, DSI has typicallybeen done in the past at temperatures much lower than 400° F. becauseequipment material, such as baghouse media, cannot withstand highertemperatures. Additionally, many sorbent materials sinter or melt attemperatures near or greater than 400° F., which makes them lesseffective at removing gases. The reactions products of many sorbentmaterials also adhere to equipment and ducts at higher temperatures,which requires frequent cleaning of the process equipment. To operate atthese lower temperatures, the combustion gases must often be cooledbefore the sorbent was injected. This is an undesirable extra processstep.

Thus, there is a need for a sorbent injection method that is effectiveat removing SO_(x) gases at elevated temperatures.

SUMMARY

In one aspect, a method of removing SO₂ from a flue gas stream includingSO₂ is provided. The method includes providing a source of trona andinjecting the trona into the flue gas stream. The temperature of theflue gas is between about 600° F. and about 900° F. The trona ismaintained in contact with the flue gas for a time sufficient to react aportion of the trona with a portion of the SO₂ to reduce theconcentration of the SO₂ in the flue gas stream.

In another aspect, a system for the removal of SO₂ from a flue gasstream including SO₂ is provided. The system includes a source of tronaand a flue gas stream. The system also includes an injector forinjecting the trona into the flue gas stream. The temperature of theflue gas is between about 600° C. and about 900° F. The system alsoincludes an area for maintaining the trona in contact with the flue gasfor a time sufficient to react a portion of the trona with a portion ofthe SO₂ to reduce the concentration of the SO₂ in the flue gas stream.

The foregoing paragraphs have been provided by way of generalintroduction, and are not intended to limit the scope of the followingclaims. The presently preferred embodiments, together with furtheradvantages, will be best understood by reference to the followingdetailed description taken in conjunction with the accompanyingdrawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic of one embodiment of a flue gas desulfurizationsystem.

FIG. 2 is a graph showing the % SO₂ removal as a function of normalizedstochiometric ratio (NSR) for trona and sodium bicarbonate.

FIG. 3 is a graph showing the % SO₂ removal as a function of flue gastemperature for one embodiment of a flue gas desulfurization system.

FIG. 4 shows a perforated plate of an electrostatic precipitator afteroperation in one embodiment of a flue gas desulfurization system usingtrona.

FIG. 5 shows a perforated plate of an electrostatic precipitator afteroperation in one embodiment of a flue gas desulfurization system usingsodium bicarbonate.

DETAILED DESCRIPTION

The invention is described with reference to the drawings in which likeelements are referred to by like numerals. The relationship andfunctioning of the various elements of this invention are betterunderstood by the following detailed description. However, theembodiments of this invention as described below are by way of exampleonly, and the invention is not limited to the embodiments illustrated inthe drawings.

Dry sorbent injection (DSI) has been used as a low cost alternative to aspray dry or wet scrubbing system for the removal of SO₂. In the DSIprocess, the sorbent is stored and injected dry into the flue duct whereit reacts with the acid gas. The present invention provides a method ofremoving SO₂ from a flue gas stream comprising SO₂, preferably byinjecting a sorbent such as trona into a flue gas stream to react withSO₂. Trona is a mineral that contains about 85-95% sodiumsesquicarbonate (Na₂CO₃.NaHCO₃.2H₂O). A vast deposit of mineral trona isfound in southwestern Wyoming near Green River. As used herein, the term“trona” includes other sources of sodium sesquicarbonate. The term “fluegas” includes the exhaust gas from any sort of combustion process(including coal, oil, natural gas, glass raw material, etc.). Flue gastypically includes SO₂ along with other acid gases such as HCl, SO₃, andNO_(x).

A schematic of the process is shown in FIG. 1. The furnace or combustor10 is fed with a fuel source 12, such as coal, and with air 14 to burnthe fuel source 12. From the combustor 10, the combustion gases areconducted to a heat exchanger or air heater 40. The outlet of the heatexchanger or air heater 40 is connected to a particulate collectiondevice 50. The particulate collection device 50 removes particles madeduring the combustion process, such as fly ash, from the flue gas beforeit is conducted to the gas stack 60 for venting. The particulatecollection device 50 may be an electrostatic precipitator (ESP). Othertypes of particulate collection devices, such as a baghouse, may also beused for solids removal. The baghouse contains filters for separatingparticles made during the combustion process from the flue gas. Becauseof the relatively small particle size used in the process, the trona mayact as a precoat on baghouse filter media.

The SO₂ removal system includes a source of trona 30. The trona 30preferably has a mean particle size between about 10 micron and about 40micron, most preferably between about 24 micron and about 28 micron. Thetrona is preferably in a dry granular form. A suitable trona source isT-200® trona, which is a mechanically refined trona ore productavailable from Solvay Chemicals, Green River, Wyo. T-200® trona containsabout 97.5% sodium sesquicarbonate and has a mean particle size of about24-28 micron. The SO₂ removal system may also include a ball millpulverizer 32, or other type of mill, for decreasing and/or otherwisecontrolling the trona particle size on site.

The trona is conveyed from the trona source 30 to the injector 20. Thetrona may be conveyed pneumatically or by any other suitable method.Trona can be easily aerated for pneumatic transfer. Apparatus forinjecting the trona or sodium sesquicarbonate is schematicallyillustrated in FIG. 1. Trona injection apparatus 20 introduces the tronainto flue gas duct section 42, which is disposed at a position upstreamof the baghouse inlet and upstream of the heat exchanger 40, if a heatexchanger or preheater is present. The trona injection system ispreferably designed to maximize contact of the trona with the SO_(x) inthe flue gas stream. Any type of injection apparatus known in the artmay be used to introduce the trona into the gas duct. For example,injection can be accomplished directly by a compressed air-driveneductor.

The process requires no slurry equipment or reactor vessel if the tronais stored and injected dry into the flue duct 42 where it reacts withthe acid gas. However, the process may also be used with humidificationof the flue gas or wet injection of the trona. Additionally, theparticulates can be collected wet through an existing wet scrubbervessel should the process be used for trim scrubbing of acid mist.

The temperature of the flue gas varies with the location in theinjection system and may also vary somewhat with time during operation.The temperature of the flue gas where the trona is injected is betweenabout 600° F. and about 900° F. The trona is maintained in contact withthe flue gas for a time sufficient to react a portion of the trona witha portion of the SO₂ to reduce the concentration of the SO₂ in the fluegas stream. The temperature of the flue gas is preferably greater thanabout 630° F., and most preferably greater than about 700° F. Thetemperature of the flue gas is preferably less than about 800° F., andmost preferably less than about 750° F. The temperature of the flue gasis most preferably between about 700° F. and about 750° F.

The process may also be varied to control the flue gas temperature. Forexample, the flue gas temperature upstream of the trona may be adjustedto obtain the desired flue gas temperature where the trona is injected.Additionally, ambient air may be introduced into the flue gas stream andthe flue gas temperature monitored where the trona is injected. Otherpossible methods of controlling the flue gas temperature include usingheat exchanges and/or air coolers. The process may also vary the tronainjection location or include multiple locations for trona injection.

For the achievement of desulfurization, trona is preferably injected ata rate with respect to the flow rate of the SO₂ to provide a normalizedstoichiometric ratio (NSR) of sodium to sulfur of between about 1.0 and1.5. The NSR is a measure of the amount of reagent injected relative tothe amount theoretically required. The NSR expresses the stoichiometricamount of sorbent required to react with all of the acid gas. Forexample, an NSR of 1.0 would mean that enough material was injected totheoretically yield 100 percent removal of the SO₂ in the inlet fluegas; an NSR of 0.5 would theoretically yield 50 percent SO₂ removal. SO₂neutralization requires two moles of sodium per one mole of SO₂ present.

Unlike sodium bicarbonate, trona does not melt at elevated temperatures.Rather, sodium sesquicarbonate undergoes rapid calcination of containedsodium bicarbonate to sodium carbonate when heated at or above 275° F.It is believed that the “popcorn like” decomposition creates a large andreactive surface by bringing unreacted sodium carbonate to the particlesurface for SO₂ neutralization. The byproduct of the reaction is sodiumsulfate and is collected with fly ash. The chemical reaction of thetrona with the SO₂ is represented below:2[Na₂CO₃.NaHCO₃.2H₂O]→3Na₂CO₃+5H₂O+CO₂Na₂CO₃+SO₂→Na₂SO₃+CO₂Na₂SO₃+1/2O₂→Na₂SO₄The solid reaction products of the trona and the SO₂ (primarily sodiumsulfate) and unreacted soda ash may be collected in an electrostaticprecipitator, or other particulate collection device. The totaldesulfurization is preferably at least about 70%, more preferably atleast about 80%, and most preferably at least about 90%.

In one embodiment, the flue gas stream further comprises S0 ₃. The tronais maintained in contact with the flue gas for a time sufficient toreact a portion of the trona with a portion of the SO₃ to reduce theconcentration of the SO₃ in the flue gas stream. SO₃ is typically morereactive with the sorbent than SO₂, so the trona would remove the SO₃first. The chemical reaction of the trona with the SO₃ is representedbelow:2[Na₂CO₃.NaHCO₃.2H₂O]→3Na₂CO₃+5H₂O+CO₂Na₂CO₃+SO₃→Na₂SO₄+CO₂

The trona injection system may also be combined with other SO_(x)removal systems, such as sodium bicarbonate, lime, limestone, etc. inorder to enhance performance or remove additional hazardous gases suchas HCl, NO_(x) and the like.

EXAMPLES

A study was done in a commercial glass plant in Verona, Calif. using ahot side electrostatic precipitator (ESP) and no baghouse. Natural gaswas used as a fuel source, and the source of sulfur was from the glassraw materials. The SO₂ concentration in the flue gas was 800 ppm. Thetrona used was T-200® from Solvay Chemicals. The trona was injected inthe duct using a compressed air blower and air lock feeder. Trona flowrates were measured by calibrating the airlock rpm with the trona weightloss in the trona storage bin. Trona feed rates varied from 50 to 211pounds/hr.

Example 1

Trona was injected into flue gas at a temperature of 750° F. at NSRvalues of 1.0, 1.2, and 1.4. FIG. 2 shows the % SO₂ removal as afunction of normalized stochiometric ratio (NSR) for trona. From thesetests it can be seen that trona yielded SO₂ removal rates of around 80%at an NSR of 1.2. FIG. 4 shows a perforated plate of an ESP in the glassplant after operation of the SO₂ removal system for five months usingtrona. It can be seen that the plate is relatively free of solidsbuildup.

Example 2

As a comparative example, sodium bicarbonate was injected under the sameconditions as Example 1 at an NSR of 1.2. The result is shown in FIG. 2.The % SO₂ removal of 72% was significantly lower than that of the tronaat the same temperature and NSR. FIG. 5 shows a perforated plate of anESP in the glass plant after operation of the SO₂ removal system usingsodium bicarbonate. It can be seen that the plate has significant solidsbuildup.

Example 3

Trona was injected into flue gas at a NSR of 1.5 in a temperature rangeof 750° F. to 805° F. FIG. 3 shows the % SO₂ removal as a function offlue gas temperature. From these tests it can be seen that trona yieldedSO₂ removal rates of up to 91% and was effective over a wide range ofelevated temperatures.

From the above experiments it can be seen that trona was more effectivethan sodium bicarbonate at removing SO₂ from a flue gas stream atelevated temperatures. Thus, the system can use less sorbent materialthan a sodium bicarbonate system to achieve the same sulfur reduction.Additionally, it can be seen that trona had good performance over a widerange of elevated temperatures. Finally, the SO₂ removal system usingtrona had much less solids buildup in the perforated plates of the ESPthan a system using sodium bicarbonate.

The embodiments described above and shown herein are illustrative andnot restrictive. The scope of the invention is indicated by the claimsrather than by the foregoing description and attached drawings. Theinvention may be embodied in other specific forms without departing fromthe spirit of the invention. Accordingly, these and any other changeswhich come within the scope of the claims are intended to be embracedtherein.

1. A method of removing SO₂ from a flue gas stream comprising SO₂,comprising: providing a source of trona, wherein the mean particle sizeof the trona is less than about 40 micron; injecting the trona into theflue gas stream, wherein the trona is injected at a rate with respect tothe flow rate of the SO₂ to provide a normalized stoichiometric ratio ofsodium to sulfur of between about 1.0 and about 1.5, further wherein thetemperature of the flue gas is between about 630° F. and about 900° F.;and maintaining the trona in contact with the flue gas for a timesufficient to react a portion of the trona with a portion of the SO₂ toreduce the concentration of the SO₂ in the flue gas stream by at leastabout 78%.
 2. The method of claim 1 wherein the mean particle size ofthe trona is between about 10 micron and about 40 micron.
 3. The methodof claim 1 wherein the mean particle size of the trona is between about24 micron and about 28 micron.
 4. The method of claim 1 wherein thetemperature of the flue gas is greater than about 700° F.
 5. The methodof claim 1 wherein the temperature of the flue gas is less than about800° F.
 6. The method of claim 1 wherein the temperature of the flue gasis less than about 750° F.
 7. The method of claim 1 wherein thetemperature of the flue gas is between about 700° F. and about 750° F.8. The method of claim 1 wherein the trona is injected as a drymaterial.
 9. The method of claim 1 further comprising milling the tronato a desired mean particle size at a location proximate the flue gasstream.
 10. The method of claim 1 further comprising collecting areaction product of the trona and the SO₂ in an electrostaticprecipitator.
 11. The method of claim 1, wherein the concentration ofthe SO₂ in the flue gas stream is reduced by at least about 80%.
 12. Themethod of claim 1, wherein the maintaining of the trona in contact withthe flue gas to reduce the concentration of the SO₂ in the flue gassubstantially avoids formation of solids buildup on process equipment.13. The method of claim 1, wherein the maintaining of the trona incontact with the flue gas to reduce the concentration of the SO₂ in theflue gas substantially avoids solids buildup in a particulate collectiondevice.
 14. A method of removing SO₂ from a flue gas stream comprisingSO₂, comprising: providing a source of trona having a mean particle sizebetween about 10 micron and about 40 micron; injecting the trona as adry granular material into the flue gas stream, wherein the trona isinjected at a rate with respect to the flow rate of the SO₂ to provide anormalized stoichiometric ratio of sodium to sulfur of between about 1.0and about 1.5, further wherein the temperature of the flue gas isbetween about 700° F. and about 800° F.; and maintaining the trona incontact with the flue gas for a time sufficient to react a portion ofthe trona with a portion of the SO₂ to reduce the concentration of theSO₂ in the flue gas stream by at least about 78%.
 15. The method ofclaim 14 wherein the mean particle size of the trona is between about 24micron and about 28 micron.
 16. The method of claim 14, wherein thetemperature of the flue gas is between about 700° F. and about 750° F.17. The method of claim 14 further comprising adjusting the flue gastemperature upstream of the trona to obtain the desired flue gastemperature where the trona is injected.
 18. The method of claim 17wherein the adjusting further comprises introducing ambient air into theflue gas stream and monitoring the flue gas temperature where the tronais injected.
 19. The method of claim 17 wherein the adjusting furthercomprises controlling the flow of a material through a heat exchanger incommunication with the flue gas.
 20. The method of claim 14, wherein theconcentration of the SO₂ in the flue gas stream is reduced by at leastabout 80%.